Re: [fetch] specify types of streams (#167)

@bblfish We're planning to provide a way to create a Request / Response from a user-created streams.

```
var controller;
var stream = new ReadableStream({start: c => controller = c});
var response = new Response(stream, ...);
assert_equals(response.body, stream);
controller.enqueue(3);
var p = response.body.getReader().read();
// |p| will be resolved with |{done: false, value: 3}|.
```

IIUC we are not going to restrict each chunk type for such cases.

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